It always hurts my eyes when I see a crixalis or Ursa getting Quelling Blade.
I'm suggesting to put a warning on the item.
When an Orb effect and a Demolish based ability would affect a unit which is an allowed target for both abilities the Orb effect overrides the Demolish skill fully, so no bonus damage. This is always true except for the Orb effect Feedback. This one stacks with demolish (feedback damage isn't increased).
1.) a hero with quelling blade and sny attacks an enemy creep. -> sny works, quelling doesn't
2.) a hero with quelling blade and diffusal attacks a neutral with mana pool. -> both abilities work
3.) spirit bear lvl 4 attacks an enemy building -> demolish works because the entangle orb of spiritbear doesn't have buildings as allowed target.
My suggestion is: Add a warning.